The research proposed is aimed at characterizing quantitatively the membrane interactions and potential-sensing mechanisms of certain classes of fluorescent dyes that are particularly promising tools for monitoring membrane potential changes in heart and nerve cells. Experiments are proposed for determining whether and how these dyes perturb membrane permeability. These studies are based upon the effects of these dyes in altering the ion permeability of lipid bilayer membranes. In addition, spectroscopic studies are proposed, in bilayer membrane systems for characterizing how variations in membrane composition and aqueous ionic strength alter the fluorescence responses of these dyes to a change in membrane potential. A model is proposed, based upon the results of preliminary experiments, to describe the interactions of these dyes with membranes, and this model is used to rationalize differences which have been reported for dye responses in heart and nerve membranes. In addition, preliminary results are presented which suggest a method for "calibrating" the fluorescence responses of these dyes so that they can be used as "probes" of the absolute values of transmembrane potentials in any type of cell.